The incorporation and removal of large amounts of strontium by physiologic mechanisms in mineralized tissues

Quantitative imaging of trace elements in solid samples by online isotope dilution laser ablation-inductively coupled plasma-mass spectrometry

For quantitative visualization of trace elements, an online isotope dilution (ID) laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) method capable of accurately correcting the detection amount of targets is presented. Two aerosols from an ablated sample and a nebulized isotopically enriched spike solution were mixed online using a dual-port spray chamber. In this paper, transmission efficiency (TE) of each of the two gas streams to the ICP-MS detector is revealed, and the quantification values were corrected by including the ratio of TEs in the calculation of the online ID method. To verify the developed method, Fe and Sr as model elements in five certified reference materials (CRMs, glass: SRM 610, 612, 614, alloy: SS-356, -383) were quantified without the use of matrix-matched CRMs. The resultant values agreed with the certified values of CRMs in the range of 92.7-104.7% and 92.8-109.0% for Fe and Sr, respectively. The LODs (3σ) were 0.54 and 0.17 μg g-1 for Fe and Sr, respectively. In addition, the applicability of this method to quantitative imaging of unknown solid samples was demonstrated for actual biological hard tissues (a mouse incisor, human primary tooth, and fish otolith) using the result of shot-analysis. The results were consistent with the reported concentration range obtained by wet chemical analysis.

Strontium Functionalization of Biomaterials for Bone Tissue Engineering Purposes: A Biological Point of View

Strontium (Sr) is a trace element taken with nutrition and found in bone in close connection to native hydroxyapatite. Sr is involved in a dual mechanism of coupling the stimulation of bone formation with the inhibition of bone resorption, as reported in the literature. Interest in studying Sr has increased in the last decades due to the development of strontium ranelate (SrRan), an orally active agent acting as an anti-osteoporosis drug. However, the use of SrRan was subjected to some limitations starting from 2014 due to its negative side effects on the cardiac safety of patients. In this scenario, an interesting perspective for the administration of Sr is the introduction of Sr ions in biomaterials for bone tissue engineering (BTE) applications. This strategy has attracted attention thanks to its positive effects on bone formation, alongside the reduction of osteoclast activity, proven by in vitro and in vivo studies. The purpose of this review is to go through the classes of biomaterials most commonly used in BTE and functionalized with Sr, i.e., calcium phosphate ceramics, bioactive glasses, metal-based materials, and polymers. The works discussed in this review were selected as representative for each type of the above-mentioned categories, and the biological evaluation in vitro and/or in vivo was the main criterion for selection. The encouraging results collected from the in vitro and in vivo biological evaluations are outlined to highlight the potential applications of materials’ functionalization with Sr as an osteopromoting dopant in BTE.

Exposure to e-cigarette aerosol over two months induces accumulation of neurotoxic metals and alteration of essential metals in mouse brain

Despite a recent increase in e-cigarette use, the adverse human health effects of exposure to e-cigarette aerosol, especially on the central nervous system (CNS), remain unclear. Multiple neurotoxic metals have been identified in e-cigarette aerosol. However, it is unknown whether those metals accumulate in the CNS at biologically meaningful levels. To answer this question, two groups of mice were whole-body exposed twice a day, 5 days a week, for two months, to either a dose of e-cigarette aerosol equivalent to human secondhand exposure, or a 5-fold higher dose. After the last exposure, the olfactory bulb, anterior and posterior frontal cortex, striatum, ventral midbrain, cerebellum, brainstem, remaining brain tissue and spinal cord were collected for metal quantification by inductively coupled plasma mass spectrometry and compared to tissues from unexposed control mice. The two-month exposure caused significant accumulation of several neurotoxic metals in various brain areas - for some metals even at the low exposure dose. The most striking increases were measured in the striatum. For several metals, including Cr, Cu, Fe, Mn, and Pb, similar accumulations are known to be neurotoxic in mice. Decreases in some essential metals were observed across the CNS. Our findings suggest that chronic exposure to e-cigarette aerosol could lead to CNS neurotoxic metal deposition and endogenous metal dyshomeostasis, including potential neurotoxicity. We conclude that e-cigarette-mediated metal neurotoxicity may pose long-term neurotoxic and neurodegenerative risks for e-cigarette users and bystanders.

Strontium Uptake and Intra-Population 87Sr/86Sr Variability of Bones and Teeth—Controlled Feeding Experiments With Rodents (Rattus norvegicus, Cavia porcellus)

Strontium isotopes in biogenic apatite, especially enamel, are widely employed to determine provenance and track migration in palaeontology and archaeology. Body tissues record the 87Sr/86Sr of bioavailable Sr of ingested food and water. To identify non-local individuals, knowledge of the 87Sr/86Sr of a non-migratory population is required. However, varying factors such as tissue turnover rates, feeding selectivity, Sr content, digestibility of food, and the ingestion of mineral dust can influence body tissue 87Sr/86Sr. To evaluate the Sr contribution of diet and water to mammalian hard tissues 87Sr/86Sr, controlled feeding studies are necessary. Here we present 87Sr/86Sr from controlled feeding experiments with two rodent species (Rattus norvegicus, Cavia porcellus). Due to the continuous and fast incremental growth of rat and guinea pig incisors (~0.1 – 0.5 mm/day), their enamel is expected to record isotopic dietary changes. For Experiment-1: Diet Switch, animals were switched from their respective supplier food to a pelleted experimental diet containing either insect-, plant-, or meat-meal and a staggered-sampling approach was used to monitor the 87Sr/86Sr changes in rat incisor enamel and bone over the course of the experiment. In Experiment-2: Basic Diets, separated cohorts (n = 6) of rats and guinea pigs were fed one of the three pelleted diets and received tap water for 54 days. While the rat incisors showed a complete tissue turnover, the slower-growing guinea pig incisors partially retained supplier diet-related isotopic compositions. In addition, one group of rats fed plant-meal pellets received Sr-rich mineral water, demonstrating that drinking water can be an important Sr source in addition to diet. Additionally, a leaching experiment showed that only a small fraction of diet-related Sr is bioavailable. Finally, in Experiment-3: Dust Addition, guinea pigs were fed pellets with and without addition of 4% of isotopically distinct dust (loess or kaolin). Animals that received kaolin-containing pellets displayed increased enamel 87Sr/86Sr. Intra-population 87Sr/86Sr variability within each feeding group was small and thus we conclude that it should not affect interpretations of 87Sr/86Sr in provenance studies. However, the differences between bulk food and leachate 87Sr/86Sr highlight the importance of Sr bioavailability for provenance studies and Sr isoscapes.

Pulsed laser deposition temperature effects on strontium-substituted hydroxyapatite thin films for biomedical implants

Substituting small molecule drugs with abundant and easily affordable ions may have positive effects on the way countless disease treatments are approached. The interest in strontium cation in bone therapies soared in the wake of the success of strontium ranelate in the treatment of osteoporosis. A new method for producing thin strontium-containing hydroxyapatite (Sr-HA, Ca₉Sr(PO₄)₆(OH)₂) films as coatings that render bioinert titanium implant bioactive is reported here. The method is based on the combination of a mechanochemical synthesis of Sr-HA targets and their deposition in form of thin films on top of titanium with the use of laser ablation at low pressure. The films were 1-2 μm in thickness and their formation was studied at different temperatures, including 25, 300, and 500 °C. Highly crystalline Sr-HA target transformed during pulsed laser deposition to a fully amorphous film, whose degree of long-range order recovered with temperature. Particle edges became somewhat sharper and surface roughness moderately increased with temperature, but the (Ca+Sr)/P atomic ratio, which increased 1.5 times during the film formation, remained approximately constant at different temperatures. Despite the mostly amorphous structure of the coatings, their affinity for capturing atmospheric carbon dioxide and accommodating it as carbonate ions that replace both phosphates and hydroxyls of HA was confirmed in an X-ray photoelectron spectroscopic analysis. As the film deposition temperature increased, the lattice voids got reduced in concentration and the structure gradually "closed," becoming more compact and entailing a linear increase in microhardness with temperature, by 0.03 GPa/°C for the entire 25-500 °C range. Biocompatibility and bioactivity of Sr-HA thin films deposited on titanium were confirmed in an interaction with dental pulp stem cells, suggesting that these coatings, regardless of the processing temperature, may be viable candidates for the surface components of metallic bone implants.

Strontium stimulates alkaline phosphatase and bone morphogenetic protein-4 expression in rat chondrocytes cultured in vitro

The trace element strontium has a significant impact on cartilage metabolism. However, the direct effects of strontium on alkaline phosphatase (ALP), a marker of bone growth, and bone morphogenetic protein-4 (BMP-4), which plays a key role in the regulation of bone and cartilage development, are not entirely clear. In order to understand the mechanisms involved in these processes, the chondrocytes were isolated from Wistar rat articular cartilage by enzymatic digestion and cultured under standard conditions. They were then treated with strontium at 0.5, 1.0, 2.0, 5.0, 20.0 and 100.0 μg/mL for 72 h. The mRNA abundance and protein expression levels of ALP and BMP-4 were measured using real-time polymerase chain reaction (real-time PCR) and Western blot analysis. The results showed that the levels of expression of ALP and BMP-4 in chondrocytes increased as the concentration of strontium increased relative to the control group, and the difference became significant at 1.0 μg/mL strontium (P<0.05). These results indicated that strontium could be involved in cartilage development via regulating ALP and BMP-4 expression.

Reconstructing Ancient Egyptian Diet through Bone Elemental Analysis Using LIBS (Qubbet el Hawa Cemetery)

One of the most important advantages of LIBS that make it suitable for the analysis of archeological materials is that it is a quasi-nondestructive technique. Archeological mandibles excavated from Qubbet el Hawa Cemetery, Aswan, were subjected to elemental analysis in order to reconstruct the dietary patterns of the middle class of the Aswan population throughout three successive eras: the First Intermediate Period (FIP), the Middle Kingdom (MK), and the Second Intermediate Period (SIP). The bone Sr/Ca and Ba/Ca ratios were significantly correlated, so the Sr/Ca ratios are considered to represent the ante-mortem values. It was suggested that the significantly low FIP Sr/Ca compared to that of both the MK and the SIP was attributed to the consumption of unusual sorts of food and imported cereals during years of famine, while the MK Sr/Ca was considered to represent the amelioration of climatic, social, economic, and political conditions in this era of state socialism. The SIP Sr/Ca, which is nearly the same as that of the MK, was considered to be the reflection of the continuity of the individualism respect and state socialism and a reflection of agriculture conditions amelioration under the reign of the 17th Dynasty in Upper Egypt.

The concentration of manganese, iron, and strontium in hip joint bone obtained from patients undergoing hip replacement surgery

The aim of this study was to determine the concentrations of manganese (Mn), iron (Fe) and strontium (Sr) in the cartilage with adjacent compact bone and spongy bone collected from patients after total hip replacement surgery. In addition, we examined relations between the concentrations of the metals in the bone and selected environmental factors. The concentration of Fe was the highest while Mn concentration was the lowest. The concentrations of Fe in the spongy bone in patients from larger cities were higher than in those living in smaller towns and villages. Significant correlations were found between Fe and Mn concentrations in the cartilage with adjacent compact bone and in the spongy bone, and between Mn and Sr in the spongy bone. In general, Mn, Fe and Sr concentrations in the bones of patients from NW Poland were lower than in other Polish regions and Europe, especially in industrialized countries. In conclusion, it seems that in addition to routine monitoring of the abiotic environment, it is essential to monitor concentrations of heavy metals having a long-term impact in humans.

Dietary strontium increases bone mineral density in intact zebrafish (Danio rerio): a potential model system for bone research

Zebrafish (Danio rerio) skeletal bone possesses properties similar to human bone, which suggests that they may be used as a model to study mineralization characteristics of the human Haversian system, as well as human bone diseases. One prerequisite for the use of zebrafish as an alternative osteoporotic bone model is to determine whether their bone displays functional plasticity similar to that observed in other bone models. Strontium citrate was supplemented into a laboratory-prepared diet (45% crude protein) to produce dietary strontium levels of 0%, 0.63%, 1.26%, 1.89%, and 2.43% and fed ad libitum twice daily for 12 weeks to 28-day-old intact zebrafish. Length was determined at 4-week intervals, and both weight and length were recorded at 12 weeks. At 12 weeks, seven zebrafish from each dietary level were analyzed for total bone mineral density by microcomputed tomography. Dietary strontium citrate supplementation significantly (p < 0.05) increased zebrafish whole-body and spinal column bone mineral density. In addition, trace amounts of strontium were incorporated into the scale matrix in those zebrafish that consumed strontium-supplemented diets. These findings suggest that zebrafish bone displays plasticity similar to that reported for other bone models (i.e., rat, mouse, and monkey) that received supplements of strontium compounds and zebrafish should be viewed as an increasingly valuable bone model.

Synthesis and characterization of poly(methyl methacrylate)-based experimental bone cements reinforced with TiO2-SrO nanotubes

In an attempt to overcome existing limitations of experimental bone cements we here demonstrate a simple approach to synthesizing strontium-modified titania nanotubes (n-SrO-TiO(2) tubes) and functionalize them using the bifunctional monomer methacrylic acid. Then, using 'grafting from' polymerization with methyl methacrylate, experimental bone cements were produced with excellent mechanical properties, radiopacity and biocompatibility. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy mapping and backscattered SEM micrographs revealed a uniform distribution of SrO throughout the titanium matrix, with retention of the nanotubular morphology. Nanocomposites were then reinforced with 1, 2, 4 and 6 wt.% of the functionalized metal oxide nanotubes. Under the mixing and dispersion regime employed in this study, 2 wt.% appeared optimal, exhibiting a more uniform dispersion and stronger adhesion of the nanotubes in the poly(methyl methacrylate) matrix, as shown by TEM and SEM. Moreover, this optimum loading provided a significant increase in the fracture toughness (K(IC)) (20%) and flexural strength (40%) in comparison with the control matrix (unfilled) at P<0.05. Examination of the fracture surfaces by SEM showed that toughening was provided by the nanotubes interlocking with the acrylic matrix and crack bridging during fracture. On modifying the n-TiO(2) tubes with strontium oxide the nanocomposites exhibited a similar radiopacity to a commercial bone cement (CMW 1), while exhibiting a significant enhancement of osteoblast cell proliferation (242%) in vitro compared with the control at P<0.05.

Advances in the diagnosis and treatment of osteoporosis

Although severely low bone density is relatively rare in the pediatric population, it can be a significant problem in many patients with chronic illness. As peak bone formation occurs during adolescence, it is crucial that pediatricians and other care providers for this patient population recognize the significance of attainment of adequate bone. Dietary intake of vitamin D and calcium should be optimized, and correction of underlying causes of poor bone density should occur whenever possible. Assessment of bone density is difficult, as each technology available has problems, and none of the technologies are well-associated with fracture risk in pediatric patients. Once diagnosis of severely low bone density is established, treatment options are limited and poorly studied. The benefits of bisphosphonate therapy appear to outweigh the risks in patients with low bone density and frequent fragility fractures, and it appears that most improvement with bisphosphonates occurs within the first 2 to 4 years. Evidence, however, is emerging that once off therapy, bone turnover remains decreased for at least several years. During that time, improvements in bone density are decreased. Many questions remain regarding duration of therapy with bisphosphonate therapy and the long-term effects on the children who receive this medication. Anabolic therapies may become important in the future, but there is currently extremely limited information regarding their use in pediatrics.

Time-evolution and reversibility of strontium-induced osteomalacia in chronic renal failure rats

BACKGROUND

Patients with impaired renal function can accumulate strontium in the bone, which has been associated with the development of osteomalacia. A causal role for strontium in the development of the disease was presented in chronic renal failure (CRF) rats. Strontium-ranelate has been put forward as a therapeutic agent in the treatment of osteoporosis. Since the target population for strontium treatment consists mainly in postmenopausal osteoporotic women, who may have a reduced renal function, the risk for osteomalacia should be considered.

METHODS

To determine the time evolution and reversibility of the strontium-induced mineralization defect, CRF rats were loaded with strontium (2 g/L) (+/- 200 mg/kg/day) during 2, 6, and 12 weeks, followed by a washout period of 0, 2, 4, or 8 weeks.

RESULTS

Histologic examination of the bone of the animals treated with strontium revealed signs of osteomalacia already after 2 weeks. Animals that received strontium during 6 and 12 weeks had a significantly higher osteoid perimeter, area and thickness as compared to CRF controls. After 12 weeks, the mineralization was significantly affected, as evidenced by a lower double-labeled surface, mineral apposition and bone formation rate in combination with an increased osteoid maturation time and mineralization lag time. The osteoblast perimeter was significantly lower in the strontium-treated animals. After the washout periods, these effects were reversed and the bone lesions evolved to the values of CRF controls. This went along with an 18% reduction of the bone strontium content. A significant rise in serum alkaline phosphatase (ALP) activity was apparent in the strontium-treated animals as compared to CRF controls. This was not only due to higher levels of the bone ALP but also to those of the liver and the intestinal isoenzymes. Serum parathyroid hormone (PTH) levels decreased during strontium treatment. After cessation of the treatment, the serum ALP activity and PTH concentration reversed to control levels.

CONCLUSION

In this study evidence is provided for the rapid development of a mineralization defect in strontium-loaded CRF rats, accompanied by a reduced osteoblast number, reduced PTH synthesis or secretion, and increased serum ALP levels. These effects can be rapidly reversed after withdrawal of the compound.

In vivo cancellous bone remodeling on a strontium-containing hydroxyapatite (sr-HA) bioactive cement

The purpose of this study was to investigate the in vivo bone response to the strontium-containing hydroxyapatite (Sr-HA) bioactive bone cement injected into the cancellous bone. Sr-HA cement was injected into the iliac crest of rabbits for 1, 3, and 6 months. Active bone formation and remodeling were observed after 1 month. Newly formed bone was observed to grow onto the bone cement after 3 months. Thick osteoid layer with osteoblasts formed along the bone and guided over the bone cement surface reflected the stimulating effect of Sr-HA. From scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis, high calcium and phosphorus levels were detected at the interface with a thick layer of 70 microm in width, and fusion of Sr-HA with the bone was observed. Blood vessels were found developing in remodeling sites. The affinity of bone on Sr-HA cement was increased from 73.55 +/- 3.50% after 3 months up to 85.15 +/- 2.74% after 6 months (p < 0.01). In contrast to Sr-HA cement, poly(methyl methacrylate) (PMMA) bone cement was neither osteoconductive nor bioresorbable. Results show that the Sr-HA cement is biocompatible and osteoconductive, which is suitable for use in treating osteoporotic vertebral fractures.

New anabolic therapies in osteoporosis

Anabolic agents represent an important new advance in the therapy of osteoporosis. Their potential might be substantially greater than the anti-resorptives. Because the anti-resorptives and anabolic agents work by completely distinct mechanisms of action, it is possible that the combination of agents could be significantly more potent than either agent alone. Recent evidence suggests that a plateau in BMD might occur after prolonged exposure to PTH. Anti-resorptive therapy during or after anabolic therapy might prevent this skeletal adaptation. Protocols to consider anabolic agents as intermittent recycling therapy would be of interest. Of all the anabolics, PTH is the most promising. However, there are unanswered questions about PTH. More studies are needed to document an anabolic effect on cortical bone. More large-scale studies are needed to further determine the reduction in nonvertebral fractures with PTH, especially at the hip. In the future, PTH is likely to be modified for easier and more targeted delivery. Oral or transdermal delivery systems may become available. Recently, Gowen et al have described an oral calcilytic molecule that antagonizes the parathyroid cell calcium receptor, thus stimulating the endogenous release of PTH. This approach could represent a novel endogenous delivery system for intermittent PTH administration. Rising expectations that anabolic therapies for osteoporosis will soon play a major role in treating this disease are likely to fuel further studies and the development of even more novel approaches to therapy.

New anabolic therapies in osteoporosis

While antiresorptive drugs have been the cornerstone of osteoporosis therapy, anabolic drugs are an important new advance in the treatment of osteoporosis. By directly stimulating bone formation, anabolic agents might have greater potential than the antiresorptives to increase bone mass and to decrease fractures. It is also possible that the combination of an antiresorptive agent with an anabolic agent could be more potent than either agent alone. Potential anabolic therapies for osteoporosis, including fluoride, growth hormone, insulin-like growth factor-I, strontium, and parathyroid hormone, are reviewed here. Of these, parathyroid hormone has clearly emerged as the most promising treatment at this time.

Strontium increases vertebral bone volume in rats at a low dose that does not induce detectable mineralization defect

Low doses of strontium and fluoride were shown to increase bone formation and trabecular bone density in rodents. To assess whether strontium or fluoride affect the quality of the mineral at doses known to increase bone density, we have determined the effects of low doses of strontium and fluoride on bone formation and bone mineral characteristics in rats. Adult rats were given strontium alone (0.20%), fluoride alone (1 mg/kg per day), or the combined treatment for 8 weeks. Strontium levels in serum and femur were similar in groups treated with strontium alone or in combination, being about 5% of calcium levels. Biochemical and neutron activation analyses in femur showed that calcium and magnesium contents did not differ in the four group of rats, suggesting that strontium was incorporated in the apatite lattice of the bone minerals in the strontium-treated rats. The mineralized bone volume was significantly increased by 17% in the strontium-treated group, by 20% in the fluoride-treated group, and by 19% in rats given with the combined treatment. This was associated with increased osteoid surface, osteoblast surface, and double tetracycline labeled surfaces in the strontium-treated and fluoride-treated groups, showing that the number of bone forming sites was increased. However, the mineral apposition rate, the osteoid thickness, and the mineralization lag time were similar in controls and treated groups, reflecting the lack of deleterious effects of low doses of strontium and fluoride on bone mineralization. The density fractionation analysis measured in the femur also showed that neither strontium, nor fluoride at the low doses used, significantly altered the mineralization profile. The results indicate that treatment with low doses of strontium or fluoride increase the number of bone forming sites and vertebral bone volume in rats, but does not have detectable adverse effects on the mineral profile, bone mineral chemistry or bone matrix mineralization.

Changes in the plasma electrolytes and metabolites of the rat following acute exposure to sodium fluoride and strontium chloride

Acute exposure of rats to strontium or fluoride by i.p. injection of sodium fluoride or strontium chloride resulted in a systemic response in which changes occurred in the plasma electrolytes and metabolites. Strontium resulted in a rapid but temporary hypercalcaemia while fluoride produced a temporary hypocalcaemia. There was no significant hypophosphataemia after fluoride and only a transient hypophosphataemia with strontium. There was some indication of kidney damage and a general stress response following fluoride injection. These results do not support the hypothesis that interglobular dentine is associated with hypophosphataemia or hypoplastic enamel with hypocalcaemia and are in conflict with the observation that the formation of interglobular dentine following the injection of lead acetate is associated with hyperphosphataemia and hypercalcaemia.

Effects of low doses of strontium on bone quality and quantity in rats

Strontium (Sr) has been shown to increase bone mass when given at low doses. In this study, the diets of rats containing 0.50% calcium were supplemented with Sr (0.19 and 0.40% of SrCl2 orally) for periods of four and eight weeks. Long bones and vertebrae were studied by density fractionation and each fraction was analyzed chemically. X-ray diffraction was used to determine crystal size. Static and dynamic histomorphometric parameters of bone formation were also measured. We found a shift towards lower density in the mineralization profiles of Sr supplemented rats (0.40%), as well as a decrease in bone crystal size at the larger dose of Sr. The CO3 content and the Ca/Sr ratio of the bone decreased with increasing Sr content. We found an increase in the vertebral trabecular bone volume together with an increase in osteoid volume in Sr supplemented rats. This study shows that Sr at the larger dose induces bone hypomineralization as well as an increase in bone mass in rats fed a relatively low calcium diet.

The effect of strontium, cobalt and fluoride on rat incisor enamel formation

This investigation examined ultrastructurally the entire period of development of alterations in formative ameloblasts and the enamel which they produce following injection with fluoride, strontium, and cobalt ions. Rats injected with these ions were sacrificed at intervals of 1, 2, 4, 8, 16, 24 and 48 hours to elucidate the sequence and detail of cytologic and cell product alterations which occur. Undecalcified sections of rat incisor teeth were studied using electron microscopy and microradiography. All three ions initially produced disturbances in cell morphology and enamel formation consisting of dark globules, vacuoles, and pooling of stippled material on the enamel surface. While a period of decreased crystal formation occurred after injection with all three ions, only cobalt responses included a period of apparently complete absence of crystal formation. The hypermineralized layers occurring in the altered enamel are attributed to changes in the rate of enamel matrix formation and duration of its exposure to tissue fluids. Morphologic changes in Tomes' process were observed at the time of formation of abnormel following injection of all three ions. These observations are compared with previous studies of altered enamel formation and analyzed with the goal of learning more about the mechanisms of amelogenesis.

Strontium, calcium, magnesium, and phosphorus content of rat incisors as determined by electron microprobe analysis

Divalent cation and phosphorus content of normal and strontium-laden rat maxillary incisors was determined by electron microprobe analysis. A high positive correlation of magnesium and strontium distributions was found. Where strontium was elevated, calcium was only slightly depressed. Phosphorus appeared to be distributed randomly.